1. Field of the Invention
This invention relates generally to the transmission of data over optical fibers and, more particularly, to the use of frequency peaking in transmitters to improve the performance of the transmission.
2. Description of the Related Art
Optical fiber is widely used as a communications medium in high speed digital networks, including local area networks (LANs), storage area networks (SANs), and wide area networks (WANs). There has been a trend in optical networking towards ever-increasing data rates. While 100 Mbps was once considered extremely fast for enterprise networking, attention has recently shifted to 10 Gbps, 100 times faster. As used in this disclosure, 10 Gigabit (abbreviated as 10 G or 10 Gbps) systems are understood to include optical fiber communication systems that have data rates or line rates (i.e., bit rates including overhead) of approximately 10 Gigabits per second.
Regardless of the specific data rate, application or architecture, communications links (including optical fiber communications links) invariably include a transmitter, a channel and a receiver. In an optical fiber communications link, the transmitter typically converts the digital data to be sent to an optical form suitable for transmission over the channel (i.e., the optical fiber). The optical signal is transported from the transmitter to the receiver over the channel, possibly suffering channel impairments along the way, and the receiver then recovers the digital data from the received optical signal.
A typical 10 G optical fiber communications link 100 is shown in FIG. 1. The link 100 include a transmitter 105 coupled through optical fiber 110 (the channel) to a receiver 120. A typical transmitter 105 may include a serializer, or parallel/serial converter (P/S), 106 for receiving 10 G data from a data source on a plurality of parallel lines and providing serial data to a 10 G laser driver 107. The laser driver 107 then drives a 10 G laser 108 which launches an optical signal carrying the data on optical fiber 110.
A typical receiver 120 includes a 10 G photodetector 111 for receiving and detecting data from the fiber 110. The detected data is typically processed through a 10 G transimpedance amplifier 112, a 10 G limiting amplifier 113, and a 10 G clock and data recovery unit 114. The data may then be placed on a parallel data interface through a serial/parallel converter (S/P) 115.
In an optical fiber communications system, the optical power output by a laser is commonly modulated in a binary fashion to send data over an optical fiber. Nominally, the optical power is high for the duration of a bit period to send a logical “1,” and low to send a logical “0.” This is commonly referred to as on-off-keying, where “on” means high laser power and “off” means low laser power. In nonreturn-to-zero (NRZ) modulation, the output power stays at nominally the same level for an entire bit period. In actuality, the level is not perfectly constant for the entire bit period due to various effects. Nonetheless, a common feature of NRZ modulation is that a long string of zeros or a long string of ones will each result in an optical signal that tends to a constant steady state value.
Standards play an important role in networking and communications. Since components in the network may come from different vendors, standards ensure that different components will interoperate with each other and that overall system performance metrics can be achieved even when components are sourced from different vendors. There are a number of standards that relate to 10 G fiber networks. For example, the IEEE 802.3aq committee is developing a new standard (10GBASE-LRM) for 10 G Ethernet over multi-mode fiber over distances of up to 220 meters using electronic dispersion compensation (EDC). This standard is in a draft state, currently documented in IEEE Draft P802.3aq/D3.1, Draft amendment to: IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements, Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, Amendment: Physical Layer and Management Parameters for 10 Gb/s Operation, Type 10GBASE-LRM, which is incorporated herein by reference.
Standards committees define agreed-upon metrics to quantify performance of various components of the system being standardized. For example, in the case of optical fiber communications link, a quantity known as Optical Modulation Amplitude (OMA) is often used to characterize the signal strength of the transmitted optical waveform. OMA is the difference in optical power for the nominal “1” and “0” levels of the optical signal. Another parameter used to characterize an optical fiber transmission is the extinction ratio (ER), which is the ratio of the nominal “1”optical power level to the nominal “0”optical power level. Techniques for measuring both OMA and ER are defined in the IEEE 802.3aq draft standard. The technique defined by 802.3aq to measure OMA is to capture samples of a test waveform using a sampling oscilloscope. The test waveform is a square waveform consisting of several consecutive 1's followed by several consecutive 0's in a repeating pattern. The mean optical power level of an optical 1 is measured over the center 20% of the time interval where the signal is high, and similarly for 0's when the signal is low. The frequency of the square wave used to measure the OMA will be referred to as the “OMA measurement frequency.” The OMA quantity as a measure of transmitter power can be used to normalize performance metrics relative to a standardized transmitter power.
One such performance metric is the optical power penalty. Assume that some signal quality effect causes a drop in the signal to noise ratio of a certain amount. The impact of that effect can be characterized by an optical power penalty. The optical power penalty is the decrease in optical power (e.g., as measured by OMA in certain cases) that would result in the same drop in the signal to noise ratio. All else being equal, a lower power penalty means a better signal to noise ratio (and better performance) than a higher power penalty.
One measure of optical power penalty is referred to as PIED (Penalty for Ideal Equalizer—DFE). See for example, S. Bhoja, “Channel metrics for EDC-based 10GBASE-LRM ,” IEEE 802.3aq Task Force, July 2004, available online at:    http://grouper.ieee.org/groups/802/3/aq/public/jul04/bhoja 1 0704.pdf which is incorporated herein by reference. PIED is a calculation for optical power penalty for a type of EDC known as decision feedback equalization (DFE) and is given in optical dB by
                              PIE          D                =                  5          ⁢                                                    log                10                            ⁡                              (                                  exp                  ⁡                                      (                                                                  -                        2                                            ⁢                      T                      ⁢                                                                        ∫                          0                                                      1                                                          2                              ⁢                              T                                                                                                      ⁢                                                                              ln                            ⁡                                                          (                                                                                                                                    1                                    T                                                                    ⁢                                                                                                                                                                                                                                    H                                          a                                                                                ⁡                                                                                  (                                          f                                          )                                                                                                                                                                                            2                                                                                                  +                                                                  σ                                  2                                                                                            )                                                                                ⁢                                                                                                          ⁢                                                      ⅆ                            f                                                                                                                )                                                  )                                      .                                              (        1        )            
PIED is the ratio of two signal-to-noise ratios (SNRs). The first signal-to-noise ratio is a matched filter bound SNR, SNRMFB-Rect, which is the SNR of a matched filter receiver that receives a perfect rectangular non-return-to-zero (NRZ) pulse. The second SNR is SNRDFE, which is the SNR realized at the slicer of an ideal infinite-length DFE receiver assuming that the channel can be modeled as linear. T is the bit period (1/line rate), σ is 1 /SNRMFB-Rect, and |Ha(f)|2 is the folded spectrum defined by
                                                                                    H                a                            ⁡                              (                f                )                                                          2                -                              1            T                    ⁢                                    ∑                              n                =                                  -                  ∞                                            ∞                        ⁢                                                  ⁢                                                                            H                  ⁡                                      (                                          f                      +                                              n                        T                                                              )                                                                              2                                                          (        2        )            where H(f) is the Fourier Transform of h(t), and h(t) is the response of the normalized channel to a rectangular pulse of duration T and amplitude 1. In this sense the normalized channel includes filtering by the transmitter, the fiber channel, and the front-end filter of the optical receiver. Therefore, h(t) is the convolution of a rectangular pulse with the impulse responses of filters characterizing those elements. The channel is normalized such that H(0) is equal to T. Both SNRMFB-Rect and SNRDFE are computed assuming that the minimum OMA allowed by the standard is transmitted (which effectively determines σ in Eqn. 1).
PIED is the optical power penalty corresponding to a given channel response H(f) assuming that the channel is linear. Other penalty measures include, for example, the transmit waveform and dispersion penalty (TWDP) as defined in the IEEE 802.3aq draft standard and described further in N. Swenson et al., “Explanation of IEEE 802.3, clause 68 TWDP,” available online at    http://ieee802.org/3/aq/public/tools/TWDP.pdf, which is incorporated herein by reference. The TWDP test is a compliance test for a transmitter and computes a penalty similar to that computed by PIE-D for a reference fiber channel and reference receiver. TWDP differs from PIED in that TWDP does not assume that the transmitter is linear and it uses a finite-length equalizer in the equalizing receiver.
While in theory it may be possible to overcome a power penalty by simply increasing the transmitted OMA, it has been observed in practice that when PIED or TWDP exceeds a certain value, reliable communication with a practical receiver is not possible, regardless of the transmitted OMA. For this reason, the 802.3aq committee placed an upper bound on the allowable TWDP for a compliant transmitter. Since TWDP and PIED are both based on normalized OMA, these penalties cannot be reduced by increasing the OMA of the transmitter. A conventional method of reducing the transmitter power penalty (TWDP) is to improve the quality of the transmitted signal such that the signal more closely approximates a perfect rectangular NRZ waveform. This, however, can add significant cost to the transmitter. Therefore, a need exists to reduce the transmitter power penalty (and other penalties) in a cost effective manner, thus increasing the performance of the communications link.